Much recent work has been done in using materials other than Silicon in the manufacture of semiconductor devices. In particular experiments have been conducted on GaAs and GaN process technologies in attempts to use solid state technology in ultra high voltage applications. However, the prior tests have found limitation in these devices in withstanding ESD events due to avalanche breakdown. A typical GaAs device 100 is shown in FIG. 1, which has a drain contact 120 and a source contact 122, separated by a GaN channel region 108 over which is formed an AlN layer 110 in FIG. 1.
In the other prior art structure shown in FIG. 2, an AlGaN layer 130 is formed over the GaN layer 108. Thus the device forms a heterojunction device having a high electron mobility junction formed between two different materials having different band gaps. Due to the bandgap mismatch between the AlGaN and GaN material, the junction between the GaN channel region 108 and AlGaN cap 110 forms a conductive channel region known as a 2D electron gas (2DEG) channel. A semi-insulated AlGaN buffer 106 near the source contact becomes the site of hole accumulation due to holes generated in the channel region 108. The hole charge lowers the barrier at the channel-buffer interface, causing electrons to be injected from the source, thereby creating an avalanche region in the buffer. This causes more holes to be generated and in turn causes more electrons to be injected from the source, leading to avalanche breakdown in the buffer layer.
In one study that was conducted for a floating gate device making use of Schottky contacts to the drain and source, the drain contact was found to be functional to an Istress of 1.75 A, while in a grounded gate device, the contact degradation to the drain contact took place much sooner at an Istress of about 0.2 A, with failure at 0.45 A, probably due to increased temperature in the buffer and power dissipation in the gate region due to holes being collected by the gate. As device width increased the failure current was found to increase in a GaAs device. However GaN devices were found not to be as scalable in size in order to achieve the higher failure currents, possibly because of defects and dislocations in the material.
The present invention provides a more robust GaN device for use in ultra high voltage (UHV) applications with reversible snapback capabilities under Electrostatic Discharge (ESD) stress.